1. Field of the Invention
This invention relates to a method of producing pure and dense fused synthetic silica particles from a high purity amorphous silica powder. More particularly, the invention relates to a method of agglomerating pure silica to form coarse particles, then calcining the particles to increase density and volatilize impurities, essentially without devitrification. Particle size of the final product can be adjusted as desired by air classification and/or grinding in a jet mill. The classifier and the jet mill are lined with abrasion resistant lining such as polyurethane. Polyurethane contamination of the product can be easily eliminated by oxidation.
2. Description of Related Art
Silica is an important article of commerce useful for a variety of purposes. However, synthetic silica often has a low bulk density which typically is caused by the low specific gravity of the synthetic silica particles. Precipitated silica and silica derived from a sol-gel process have a highly porous structure which results in low specific gravity of the silica. Further, silica from these and other sources, including silica from pyrogenic techniques, tends to form fine, dust-like particles which escape during handling, causing contamination of the atmosphere and loss of product. Such particles are transported in commerce only with difficulty. Silica particles from these sources tend to resist free flow from storage containers such as silos, bins, and hoppers. Typically, mechanical assistance in the form of bin vibrators and similar devices is required in the transfer of such silica.
Various techniques for dealing with these difficulties have been disclosed. For example, U.S. Pat. No. 3,894,882 discloses a method said to achieve optimum bulk density while improving characteristics such as flowability and attrition. As taught therein, the material is wet pelletized in a tumbling drum to achieve a specified particle size distribution said to afford efficient packing of the particles. However, only minimal density improvement can be obtained when this technique is used with a highly porous material such as synthetic silica because the technique does not affect the specific gravity (bulk density) of the particles themselves which are agglomerated to form the pellet.
Compression molding of pellets also is known, but this technique does not maximize density because it does not reduce the porosity of the individual particles. Typically, binding agent and mold lubricant are utilized to ensure mutual adhesion of the particles and easy release of the pellet from the mold. For example, U.S. Pat. No. 4,256,682 discloses that silica treated with ammoniacal solution to form a dispersion is suitable for compression molding. Ammonia is evolved when the molded pellet is heated. Pellets produced by this method typically have a specific surface area of greater than about 175 m.sup.2 /g when fresh, and at least about 90 m.sup.2 /g after one week of aging.
Certain uses of silica require very high purity material. For example, silica used in the encapsulation or packaging of electronic computer chips must have extremely low levels of metal impurities, must have bulk density greater than 1 g/cm.sup.3, and must be essentially free of crystalline phases. In addition, the desired average particle size is 20 microns or less, with 100 percent smaller than 75 microns.
Typical of these uses is very large scale integrated (VLSI) microchip applications, where chip manufacturers require silica having extremely low concentrations of certain radioactive elements. For example, uranium and thorium concentrations must be of the order of less than 1 part per billion (ppb). The maximum acceptable level of ionic impurities, including cations such as boron, calcium, cobalt, chromium, copper, iron, potassium, magnesium, manganese, sodium, nickel, vanadium, and zinc, and anions containing phosphorus and sulphur, typically is less than 10 parts per million (ppm), and often is below 1 part per million. The concentration of halogens should be minimized to reduce chip corrosion and increase chip life.
Other uses for high purity silica material include precision laser optics, fiber optics, and advanced ceramics, including diffusion tubes and crucibles. These uses now are satisfied predominantly by natural silica sources such as quartz. Although natural quartz is a crystalline form of silica, such quartz is converted to "fused quartz" during the manufacturing process employed to make items such as crucibles and diffusion tubes.
A method of producing pure silica is described in U.S. Pat. No. 4,683,128. The method comprises coagulating a viscous aqueous alkali silicate solution by passing it through a spinning nozzle into a water-soluble organic medium to produce fine fibrous silica gel. The gel is treated first with an acid-containing solution, then with water, to extract impurities. The surface of the fibrous gel is cracked and crazed, i.e., is fissured, making acid and water purification treatment especially effective. Pure silica thus produced can be heat-treated at a temperature greater than about 1000.degree. C. The cracked and crazed fiber crumbles upon heating to yield fine particles of somewhat increased density (0.55 g/cm.sup.3) because the porosity is reduced by the heat treatment.
Size reduction and classification techniques typically introduce impurities to the particles. Autogeneous comminution techniques typically introduce impurities from the liner of the container or require complex apparatus, such as that disclosed in U.S. Pat. No. 1,935,344. Apparatus such as that disclosed in U.S. Pat. No. 3,184,169, wherein material is pneumatically pulverized by impingement against a surface, introduces large quantities of impurities. Contamination occurs even when both the nozzle through which particles are introduced and the interior surfaces of such an apparatus are lined with abrasion resistant material. Such contamination is greater than allowable in the above-described high-purity uses.
None of the above-described techniques are suitable for producing pure, dense, amorphous synthetic silica suitable for the above-described uses, essentially free of crystalline phases when first produced, after aging, and after fusion. Techniques such as compression molding typically require binding agents which degrade the purity of the silica and, to a certain degree, react with the molded material. Known techniques of unpressurized particle agglomeration do not achieve significant density improvement when agglomerating a porous silica. Similarly, the technique described in U.S. Pat. No. 4,683,128 is unsatisfactory for a number of reasons. Not only does the product have a low bulk density of about 0.55 g/cm.sup.3, but also the process requires a particular form of silica, i.e., a coagulated fibrous gel prepared in accordance with a complicated technique, and from which impurities have been extracted with hazardous acids.
It is an object of this invention to provide a method of producing pure and dense amorphous fused synthetic silica.
It is a further object of this invention to provide a method of agglomerating synthetic silica to form coarse particles, then calcining the particles to volatilize impurities and increase density, essentially without devitrification.
It is yet another object of this invention to provide a method of producing pure and dense amorphous fused synthetic silica without using binders which contaminate the silica.
It is still another object of this invention to provide a method of producing pure and dense amorphous fused synthetic silica with a minimum of devitrification.
It is a still further object of this invention to provide a method of producing pure and dense amphorous fused synthetic silica having a density greater than about 0.9 g/cm.sup.3 before sizing and classification.
It is a still further object of this invention to provide a method of producing coarse, pure, dense, amorphous fused synthetic silica which can be ground to produce silica suitable for electronic chip packaging, i.e., a silica with low uranium and thorium concentrations, essentially no crystalline phase, and bulk density of greater than about 1 g/cm.sup.3.